Last night British, French and American armed forces conducted co-ordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability and deter their use.
For the UK's part, four RAF Tornado GR4s launched Storm Shadow missiles at a military facility some 15 miles west of Homs, where the regime is assessed to keep chemical weapons in breach of Syria's obligations under the Chemical Weapons Convention.
While the full assessment of the strike is ongoing, we are confident of its success.
Let me set out why we have taken this action.
Last Saturday, up to 75 people, including young children, were killed in a despicable and barbaric attack in Douma, with as many as 500 further casualties.
We have worked with our allies to establish what happened. And all the indications are that this was a chemical weapons attack.
We have seen the harrowing images of men, women and children lying dead with foam in their mouths.
These were innocent families who, at the time this chemical weapon was unleashed, were seeking shelter underground in basements.
First-hand accounts from NGOs and aid workers have detailed the most horrific suffering, including burns to the eyes, suffocation and skin discolouration, with a chlorine-like odour surrounding the victims.
And the World Health Organisation has received reports that hundreds of patients arrived at Syrian heath facilities on Saturday night with "signs and symptoms consistent with exposure to toxic chemicals".
We are also clear about who was responsible for this atrocity.
A significant body of information, including intelligence, indicates the Syrian regime is responsible for this latest attack. I cannot tell you everything. But let me give an example of some of the evidence that leads us to this conclusion.
Open source accounts allege that a barrel bomb was used to deliver the chemicals.
Multiple open source reports claim that a regime helicopter was observed above the city of Douma on the evening of 7th April.
The opposition does not operate helicopters or use barrel bombs.
And reliable intelligence indicates that Syrian military officials co-ordinated what appears to be the use of chlorine in Douma on 7th April.
No other group could have carried out this attack. Indeed, Daesh for example does not even have a presence in Douma.
And the fact of this attack should surprise no-one.
We know that the Syrian regime has an utterly abhorrent record of using chemical weapons against its own people.
On 21st August 2013, over 800 people were killed and thousands more injured in a chemical attack also in Ghouta.
There were 14 further smaller scale chemical attacks prior to that summer.
At Khan Shaykhun on 4th April last year, the Syrian regime used Sarin against its people, killing around 100 with a further 500 casualties.
And based on the regime's persistent pattern of behaviour and the cumulative analysis of specific incidents, we judge it highly likely both that the Syrian regime has continued to use chemical weapons since then, and will continue to do so.
This must be stopped.
We have sought to do so using every possible diplomatic channel.
But our efforts have been repeatedly thwarted both on the ground and in the United Nations.
Following the Sarin attack in eastern Damascus back in August 2013, the Syrian regime committed to dismantle its chemical weapon programme - and Russia promised to ensure that Syria did this, overseen by the Organisation for the Prohibition of Chemical Weapons.
But these commitments have not been met.
A recent report from the Organisation for the Prohibition of Chemical Weapons has said that Syria's declaration of its former chemical weapons programme is incomplete.
This indicates that it continues to retain undeclared stocks of nerve agent or precursor chemicals - and is likely to be continuing with some chemical weapons production.
The OPCW inspectors have investigated previous attacks and on four occasions decided that the regime was indeed responsible.
And on each occasion when we have seen every sign of chemical weapons being used, any attempt to hold the perpetrators to account has been blocked by Russia at the UN Security Council, with six such vetoes since the start of 2017.
Just this week, the Russians vetoed a draft resolution that would have established an independent investigation into this latest attack - even making the grotesque and absurd claim that it was "staged" by Britain.
So we have no choice but to conclude that diplomatic action on its own will not be any more effective in the future than it has been in the past.
Over the last week, the UK government has been working intensively with our international partners to build the evidence picture, and to consider what action we need to take to prevent and deter future humanitarian catastrophes caused by chemical weapons attacks.
When the cabinet met on Thursday we considered the advice of the attorney general, the national security adviser and the chief of the defence staff - and we were updated on the latest assessment and intelligence picture.
And based on this advice we agreed that it was both right and legal to take military action, together with our closest allies, to alleviate further humanitarian suffering by degrading the Syrian regime's chemical weapons capability and deterring their use.
This was not about interfering in a civil war.
And it was not about regime change.
As I discussed with (US) President Trump and (French) President Macron, it was a limited, targeted and effective strike with clear boundaries that expressly sought to avoid escalation and did everything possible to prevent civilian casualties.
Together we have hit a specific and limited set of targets. They were a chemical weapons storage and production facility, a key chemical weapons research centre and a military bunker involved in chemical weapons attacks.
Hitting these targets with the force that we have deployed will significantly degrade the Syrian regime's ability to research, develop and deploy chemical weapons.
A year ago, after the atrocity at Khan Shaykhun, the US conducted a strike on the airfield from which the attack took place. But Assad and his regime haven't stopped their use of chemical weapons.
So last night's strikes by the US, UK and France were significantly larger than the US action a year ago, and specifically designed to have a greater impact on the regime's capability and willingness to use chemical weapons.
And this collective action sends a clear message that the international community will not stand by and tolerate the use of chemical weapons.
I also want to be clear that this military action to deter the use of chemical weapons does not stand alone.
We must remain committed to resolving the conflict at large.
The best hope for the Syrian people remains a political solution.
We need all partners - especially the regime and its backers - to enable humanitarian access to those in desperate need.
And the UK will continue to strive for both.
But these strikes are about deterring the barbaric use of chemical weapons in Syria and beyond.
And so to achieve this there must also be a wider diplomatic effort - including the full range of political and economic levers - to strengthen the global norms prohibiting the use of chemical weapons which have stood for nearly a century.
Although of a much lower order of magnitude, the use of a nerve agent on the streets of the UK in recent weeks is part of a pattern of disregard for these norms.
So while this action is specifically about deterring the Syrian regime, it will also send a clear signal to anyone else who believes they can use chemical weapons with impunity.
There is no graver decision for a Prime Minister than to commit our forces to combat - and this is the first time that I have had to do so.
As always, they have served our country with the greatest professionalism and bravery - and we owe them a huge debt of gratitude.
We would have preferred an alternative path.
But on this occasion there is none.
We cannot allow the use of chemical weapons to become normalised - either within Syria, on the streets of the UK or elsewhere.
We must reinstate the global consensus that chemical weapons cannot be used.
This action is absolutely in Britain's national interest.
The lesson of history is that when the global rules and standards that keep us safe come under threat, we must take a stand and defend them.
That is what our country has always done.
And that is what we will continue to do.